Cover index construction and updates with unit tests

[kirillt]

Index module must be well-tested to ensure there are no significant bugs: https://github.com/ARK-Builders/arklib/blob/main/src/index.rs

Test cases must be run during CI.

[kirillt]

Most of the work was done in this PR: https://github.com/ARK-Builders/arklib/pull/7

There is small follow-up (draft of test-case which should be verified and fixed if necessary):

/// TODO: this case is probably not working correctly
/// When renaming a file that has been identified as a collision during
/// build, any updates to it are lost from the UpdateIndex (?)
#[test]
fn should_update_index_after_renaming_collided_file_correctly() {
    run_test_and_clean_up(|path| {
        create_file_at(path.clone(), Some(FILE_SIZE_1), Some(FILE_NAME_1));
        create_file_at(path.clone(), Some(FILE_SIZE_1), Some(FILE_NAME_2));

        let mut actual = ResourceIndex::build(path.clone())
            .expect("Could not build index");

        assert_eq!(actual.collisions.len(), 1); 
        assert_eq!(actual.size(), 2);

        let mut name_from = path.clone();
        name_from.push(FILE_NAME_2);
        let mut name_to = path.clone();
        name_to.push(FILE_NAME_3);
        std::fs::rename(name_from, name_to)
            .expect("Should rename file successfully");

        let update = actual
            .update()
            .expect("Should update index correctly");

        assert_eq!(actual.collisions.len(), 1); 
        assert_eq!(actual.size(), 1); // TODO This size also changed
        assert_eq!(update.deleted.len(), 0); // TODO Probably this should be 1
        assert_eq!(update.added.len(), 0); // TODO also here
    });
}

More unit tests are always welcome.